1. Field of the Invention
The present invention relates to an active-type rangefinding method and apparatus for use with photographic cameras and video cameras, and more particularly to a multi-beam rangefinding method and apparatus.
2. Description of Related Art
Active-type rangefinding devices using optical triangulation are widely used in modern compact cameras. A rangefinding device of this type has a light projector and a light receiver spaced apart by a certain base line length. The light projector projects near-infrared light toward a photographic scene, and the light receiver receives light reflected from the object. The position of the reflected light incident upon the light receiver changes with object distance so that the object distance can be measured by electrically detecting the position of the incident reflected light.
The light projector comprises a light source and a projector lens for focusing light from the light source to a spot of light. The light receiver is comprised of a light receiving element and a light receiving lens. As the light source, an LED (Light emitting diode) for emitting near-infrared light is mainly used. As the light receiving element, a semiconductor PSD (position sensitive detector) is often used, which has two output terminals outputting two channel currents corresponding to the intensity and position of the incident light. By calculating the ratio or some other relationship between these two channel currents or their corresponding voltages, a signal dependent upon only the incident position of near-infrared light can be obtained.
The intensity of a spot of light reflected from an object changes with the object distance and reflectivity. Thus, the intensities of the two signals outputted from a PSD may sometimes become too large or small, which reduces measurement precision despite the use of the ratio of the two channel signals. For this reason, as described in U.S. patent application Ser. No. 07/600,821 filed on Oct. 22, 1990, a pair of gain control amplifiers are provided for each channel to obtain a signal having an adequate dynamic range. The gain of each gain control amplifier is set to the same optimum value in accordance with the output signal of each channel supplied from the PSD. Rangefinding is carried out under this condition by providing an appropriate level of the signals of the PSD, so that measurement precision can be made stable irrespective of the object distance and reflectivity.
It takes about 1.5 to 2 msec for the output signal of a gain control amplifier to become stable after the gain is changed. Before this stabilization period, the gain is unstable and an output signal from the gain control amplifier in that time will result in lower measurement precision. Furthermore, if near-infrared light from a fluorescent lamp becomes incident upon a PSD while the gain of a gain control amplifier is being set, the gain is set to a lower value than the optimum value, so that measurement with good precision is not possible.
There is also known a method of improving the precision of rangefinding by projecting light toward an object a plurality of times, supplying the output signals from the gain control amplifiers to a microcomputer, and calculating the object distance using an average value of the output signals. With this rangefinding method, an output signal of a gain control amplifier may saturate (overflow) and exceed the predetermined range, when the illumination condition of an object changes during rangefinding. In such a case, the gain of a gain control amplifier is again adjusted to perform rangefinding.
In particular, the following problem is associated with rangefinding under fluorescent lamp illumination. As is well known, a fluorescent lamp intermittently turns on and off at a frequency twice that of commercial power sources. Assuming that the frequency of a commercial power source is 60 Hz, the period F1 of turn-on/off of a fluorescent lamp is 8.3 msec as shown in FIG. 22A. Assuming that rangefinding is carried out by projecting light eighteen times at intervals F2 of 1 msec as shown in FIG. 22B, the timing of light projection will coincide at least two times with the peak of light radiation from the fluorescent lamp. At these times, the level of the output signal from the gain control amplifier exceeds the predetermined level. Under conditions of fluorescent lamp illumination, the gain adjustment and rangefinding are repeated many times, resulting in the rangefinding being disabled.
Of active-type rangefinding devices, there are multibeam type rangefinding devices wherein light is projected toward not only the central area of a photographic scene or frame but also peripheral areas, thereby to measure a plurality of distances for the object. One of the plurality of distances as determined for the respective measured areas is selected in accordance with a predetermined priority order, and the taking lens is set to a position determined by the selected distance. For example, it is determined whether an object distance at the central area of the frame is 4 to 5 m away or nearer, as a main object is more likely to be located at the central area of a photographic scene. If 4 to 5 m away or nearer, the object distance at the central area is preferentially used. If not, the nearest object distance of the peripheral areas is preferentially used. With a multi-beam type rangefinding device, a plurality of measurement data of an object at a plurality of points is obtained by projecting light a plurality of times, which is very effective in obtaining precise measurement. However, there arises the disadvantage that it takes a longer time to measure distances. This longer measurement time may cause the photographer to lose his desired photograph, because autofocus cameras generally carry out rangefinding and photographing upon depression of a single shutter button.